Battles drop minors about twice as much as majors. Gold keys give minors about three times as much as majors. Glory keys drop major and no minors, and glory packs give a few minors stones but no majors. Over the course of traiting every troop, you’ll need about four times as many minors as majors (and this is counting the Guardian troops, which use about about 1/6 to 1/8 of the majors currently needed, TOTAL, which you dont have yet, as well a bunch of mythics which also use more Major stones bit more by comparison). Historically, since traiting was added, majors have never had any value because of this severe imbalance.
Rather than an exchange format, what needs to be corrected are these drop to use ratios. They seem to be in the process of doing this with the relative costs of the stones required for Mythics, but at the current rate, the requirements for minors of a given color are still going up over twice as fast as those of majors for a given color. Getting the cost relative amount of, for example, total red major stones needed to be half the number of total red minor needed, which would match the drop ratio from battles, would require the addition of 16 red primary mythics while adding no other red primary troop in the meantime. More new troops requiring guild guardian levels of majors would have to start being introduced for the requirements ratios to match the drop ratios. Either that, or the drop ratios would need to drastically change.
At this point, I’m almost convinced the only reason for major stones’ existence is for Glory Keys, Gold Keys and battles to occasionally give null drops to people that have been playing a while.
I would, by the way, be in favor of allowing 1 to 1 exchange of major to minor for the same color only with no way to get back higher rarity stones from lower rarity. This would be roughly equivalent to having the correct drop ratios in the first place.